Method for manufacturing micromechanical structures in a device wafer

Abstract

The disclosure relates to a method for manufacturing recessed micromechanical structures in a MEMS device wafer. First vertical trenches in the device wafer define the horizontal dimensions of both level and recessed structures. The horizontal face of the device wafer and the vertical sidewalls of the first vertical trenches are then covered with a self-supporting etching mask which is made of a self-supporting mask material, which is sufficiently rigid to remain standing vertically in the location where it was deposited even as the sidewall upon which it was deposited is etched away. Recess trenches are then etched under the protection of the self-supporting mask. The method allows a spike-preventing aggressive etch to be used for forming the recess trenches, without harming the sidewalls in the first vertical trenches.

The invention claimed is: 
     
       1. A method for manufacturing micromechanical structures in a device wafer, wherein the manufactured structures include at least one level micromechanical structure which is level with regard to a horizontal face of the device wafer, and at least one recessed micromechanical structure which is recessed with regard to the horizontal face of the device wafer, and the method comprises the steps of:
 a) patterning two or more etching masks on the horizontal face of the device wafer, wherein a level etching mask defines at least the horizontal dimensions of the level micromechanical structures, and one or more recess etching masks which define at least the horizontal dimensions of the recessed micromechanical structures, 
 b) etching first vertical trenches in the regions of the device wafer which are not protected by either the level or recess etching masks, 
 c) removing one recess etching mask from the horizontal face of the device wafer, 
 d) covering the horizontal face of the device wafer and the vertical sidewalls of the first vertical trenches with a self-supporting etching mask which is made of a self-supporting material, 
 e) removing the self-supporting etching mask from horizontal surfaces of the device wafer, so that the unprotected horizontal regions from which the self-supporting etching mask and one recess etching mask were removed define the horizontal dimensions of recessed micromechanical structures, 
 f) etching second vertical trenches in the unprotected horizontal regions, whereby the depth of the second vertical trenches forms at least a part of the recess depth of a recessed micromechanical structure, and 
 g) removing the level etching mask and the self-supporting etching mask from the device wafer, 
 wherein the first vertical trenches are etched in a deep-reactive ion etch process to an initial first vertical trench depth, and the first vertical trenches are then etched deeper in a subsequent deep etching process where second vertical trenches are etched simultaneously, and 
 the height-to-width aspect ratio of the second vertical trenches is greater than 2. 
 
     
     
       2. A method according  claim 1 , wherein the material of the self-supporting mask is silicon dioxide. 
     
     
       3. A method according to  claim 1 , wherein the material of the self-supporting mask is silicon nitride. 
     
     
       4. A method according to  claim 1 , wherein the material of the self-supporting mask is a metallic material. 
     
     
       5. A method according to  claim 1 , wherein the material of the self-supporting mask is a polymer. 
     
     
       6. A method according to  claim 1 , wherein the subsequent etch process is also a deep-reactive ion etch process. 
     
     
       7. A method according to  claim 1 , wherein the number of recess etching masks is an integer N which is greater than one, and that, before proceeding to step g), the following steps f2)-f3) are repeated N−1 times to manufacture recessed micromechanical structures etched to N different recess depths:
 f2) removing one recess etching mask from the horizontal face of the device wafer, and 
 f3) etching vertical trenches in the unprotected horizontal regions, whereby the depth of the vertical trenches forms at least a part of the recess depth of a recessed micromechanical structure. 
 
     
     
       8. A method according to  claim 7 , wherein the level etching mask is a patterned thick layer of silicon dioxide, a first recess etching mask is a patterned thin layer of silicon dioxide, and a second recess etching mask is a patterned layer of photoresistive material. 
     
     
       9. A method according to  claim 1 , wherein the number of recess etching masks is an integer N which is greater than one, and that, before proceeding to step g), the following steps f2)-f3) are repeated N−1 times to manufacture recessed micromechanical structures etched to N different recess depths:
 f2) removing a part of the level etching mask from the horizontal face of the device wafer, and 
 f3) etching vertical trenches in the unprotected horizontal regions, whereby the depth of the vertical trenches forms at least a part of the recess depth of a recessed micromechanical structure. 
 
     
     
       10. A method according to  claim 9 , wherein the level etching mask is a patterned thick layer of silicon dioxide, a first recess etching mask is a patterned metallic layer, and a second recess etching mask is a patterned layer of photoresistive material.